Ion mobility spectrometers (IMS) are used to detect the presence of molecules of interest in a gas stream. This is achieved by passing a carrier gas comprising a minute concentration of a sample vapor (e.g., about ten parts per million) into a detector. The detector comprises an ionization chamber and a drift chamber. In the ionization chamber the carrier gas and the sample vapor are ionized via an ionization source, such as radioactive materials (e.g., nickel-63 or tritium).
A grid disposed at the end of the ionization chamber is normally maintained at the same potential as the walls of the ionization chamber in order to provide a space in which electrons and ions can accumulate and interact therein. When desired, (e.g., about every 20 milliseconds (ms)) the potential of the ionization chamber can be altered for a relatively short duration (e.g., about 0.10 to about 0.20 ms) to carry a desired amount of the ions from the ionization chamber into the drift chamber.
The drift chamber comprises electrodes that are disposed along its length and a collector (e.g., anode) disposed at an end that is opposite the inlet port from the ionization chamber. The electrodes can produce an electrical field within the chamber (positive or negative based on the specific molecules of interest) that causes the ions to travel from the inlet port towards the collector. Upon contact with the collector, the ion's current is detected as well as the time the ion required to pass through the drift chamber. The data from this analysis can then be compared to a library (e.g., a computer database) of known materials and the ion can be identified.
After the ionization chamber has allowed ample ions to pass into the drift chamber, the potential is reverted to that of an electrical charge free state such that additional ions can accumulate within the ionization chamber in anticipation for another measurement cycle.
IMS's have been employed in many applications for the identification of substances of interest, such as for the identification of narcotics and other contraband. This is achieved by constructing a graph of drift time versus scan index (i.e., a sequential listing of individual scans) called a plasmagram, for the sample substance and comparing the graph to those of known substances in a library (e.g., a database). However, the algorithms employed within the IMS (e.g., algorithms in the form of software, hardware, memory, and so forth) are only capable of accurately identifying substances that exhibit repeatable and linear drift time values with respect to sequential scans. Although many substances exhibit repeatable and linear drift time values, volatile chemical taggants such as those incorporated into explosives, exhibit non-linear and concentration-dependent responses, which can also be referred to as peak shifting. Peak shifting in the plasmagram may be caused by a chemical phenomenon in the detector, such as clustering of ion clouds as compounds migrate down the drift chamber. In other cases, peak shifting occurs as the result of molecular thermal instability of compounds that cause intermediate products to form, wherein the intermediate products are unstable and do not produce refined peaks, but rather a large shift in the drift time peak. There are other phenomena that may occur that are not described here that could also produce peak shifting.
As a result, there is a need in the art for methods for analyzing ion mobility spectrometry data that allows for the detection of molecules that exhibit non-linear and/or concentration dependent responses.